1. Field of the Invention
This invention relates generally to methods and apparatus for fabricating electronic devices on substrates, including integrated circuits. More particularly, the invention relates to methods and apparatus for enhancing the in-film particle performances of a copper (Cu) layer deposited on a substrate using the Ionized Metal Plasma (IMP) enhanced Physical Vapor Deposition (PVD) system and technique.
2. Background of the Related Art
In the manufacture of semiconductor devices, conductive metal contacts and interconnect lines are deposited over dielectric layers such as silicon oxide. The down sizing of circuits to speed them up has led to the preference of copper (Cu) over the previously preferred aluminum conductors of prior art devices. Copper, however, can diffuse into the lattice structure of silicon and silicon oxide substrates if applied directly to its surface. This copper diffusion can rapidly alter device performance characteristics and can also cause device leakage. Prior to copper deposition, therefore, it has been found preferable to deposit a very thin conformal layer of tantalum (Ta) or tantalum nitride (TaN) onto the substrate. This layer acts as a diffusion barrier to prevent copper diffusion into a silicon or silicon oxide substrate material.
When the substrate surface has been prepared for copper deposition, copper is deposited thereon. A preferred technique, even used as a precursor step to later copper deposition by electroplating rather than vacuum deposition, has been to form the initial or "seed" copper (Cu) conductor layer on the substrate by use of a plasma enhanced physical vapor deposition process.
In conventional physical vapor deposition (PVD), a processing chamber is typically operated at a pressure of 1-10 millitorr using an inert gas such as argon (or a mixture of gases). A target of the material to be deposited (or sputtered) such as copper, is connected to a source of DC power. The substrate being processed is mounted on a support member spaced from and generally parallel to the target. A glow discharge plasma is struck in the processing gas by the application of DC power, and the positive argon ions are attracted to the negatively charged target. Atoms of the target material are knocked loose or sputtered from the target due to the impact momentum of the impinging argon ion and its interaction with the target material structure or lattice. The particles of material sputtered from the target are generally neutral atoms or molecules. These particles are directed in a plurality of angles from the target surface, following a distribution of directions which varies as the cosine of the angle from the particle trajectory to an angle normal to the target surface. In fact, very few atoms are sputtered directly vertically or normal from the target surface.
In order to provide a more diverse or controllable angle of impingement of the sputtered particles from the target onto the substrate surface, it has been found desirable to ionize the metallic atoms or molecules prior to their impingement on the substrate. By providing metallic ions having a net positive charge, selected portions or the whole surface of the substrate being processed may have a negative bias supplied to it. The negative electric field of the substrate bias can affect the trajectory, and hence the angle of impingement, of the ionized metallic atoms onto the substrate surface. This directionality can be used to advantage in PVD processes wherein high aspect ratio (ratio of depth to width) vias or trenches are to be deposited. Without the use of ionized metallic particles, the almost omnidirectional neutral sputtered particles (i.e. having an almost isotropic trajectory distribution) can cause a build up of target material on the top portion and upper side-walls of high aspect ratio features on the substrate. This causes even fewer atoms to be deposited on the bottom surface and lower side-walls of such features, as the aspect ratio is actually increased. Eventually deposited material on the top and upper side-walls can bridge over and connect forming an undesirable void inside the feature. In addition, the deposited material can also provide a source of metallic flakes or loose metal particles as contaminates that can affect device characteristics in later processing steps of the substrate.
Metallic flakes or loose contaminant particles in the processing chamber are free to move about. During subsequent deposition processes in the substrate processing in the chamber such moveable particles can become attracted to and settle on the surface of the substrate. This can alter device characteristics and cause undesired leakage to occur across insulator boundaries. This results in circuit failures in the completed device being fabricated.
In order to provide ionization of the neutral sputtered metallic particles between the target and the substrate, a higher density of background gas at pressures in the range of about 10 to about 60 millitorr is used in the PVD processing chamber. Also, an RF (radio frequency) coil is placed in the chamber between the target surface and the substrate surface. The axis of the RF coil is placed generally perpendicular to the target surface and the substrate surface. The diameter of the RF coil is chosen so that it closely approaches that of the inside diameter of the processing cavity. The RF coil is connected to a source of RF power which, when so applied, causes ionization of the argon background gas. The neutral metallic particles passing through the RF coil interior between the target and the substrate interact with the RF field and the ionized background gas and become ionized. If the pressure in the chamber is fairly high, for example about 30 millitorr, the RF coil provides a high density plasma in the region between the target and the substrate surface. This PVD deposition apparatus and process enhanced by the use of higher pressure background gas and the RF coil is known as an ionized metal plasma (IMP) chamber and process.
The use of IMP-Cu apparatus and process to form a seed layer for subsequent electroplating processes for copper (Cu) deposition has been accepted by the semiconductor industry. One of the most difficult problems in using the IMP-Metal Deposition apparatus and process has been the in-film particle (or defect) performance of the process. Loose particles can be generated as previously described, or by other, more subtle, processes. For example sharply spiked metallic points on a coil surface can cause the temporary formation of very high electric fields (corona effect). Such fields can attract loose particles or cause ionized metal particles to be attracted from the plasma in the RF coil to the surface of the coil. Because of the transient nature of these high electric fields, such particles deposited on such sites are only loosely bound thereto and can later flake off. Heretofore, the in-film particle numbers (or adders) of greater than 80 per substrate have been measured wherein the adders are &gt;0.2 micrometers in diameter. It would be extremely valuable to provide an improved IMP-Cu apparatus and process which could significantly improve the in-film defects performance of the system.